This invention relates to the electrostatic separation of materials, and particularly to a method and apparatus for the free-fall electrostatic separation of materials.
Two common forms of electrostatic separators are triboelectric separators and corona separators. In corona separators, the particles to be separated are charged by subjecting them to the corona discharge of a high voltage corona electrode. Typically, this is accomplished by passing the particles between a corona electrode (having either a high negative charge or a high positive charge) and a grounded electrode. Current leakage from the corona electrode ionizes the molecules of air adjacent the corona electrode, with the result that particles passing between the corona electrode and the grounded electrode are subjected to ionic bombardment. Many of the ions discharge against the particles, thus causing the particles to gain a positive surface charge if the corona discharge electrode is a positive electrode, or a negative surface charge if the corona discharge electrode is a negative electrode. As it is easier to cause ionization in the vicinity of a curved electrode, corona electrodes are generally considerably curved electrodes, such as small diameter tube electrodes.
A corona separator works on the principle that the surface charge applied to different particles types discharges at a rate in accordance with the surface conductivity of the particles. The surface charge applied to highly conductive particles tends to discharge rapidly, and particles having high dielectric values will discharge at a much slower rate.
In order to effect separation in a free-fall corona separator, the particles are dropped through an electrostatic field in which the differently discharged particles are subjected to different degrees of deflection as a result of the applied coulomb forces associated with their charges.
As corona separation depends primarily on differences in the conductivity of the particles to be separated, it is primarily used to separate conductive materials from dielectric materials, but is generally not effective for separating dielectric materials from each other. An example of a free fall corona separator is disclosed in U.S. Pat. No. 3,625,360 issued to A. Schickel, which includes a corona charging electrode located above an electrostatic separation field provided by a plate capacitor.
In a triboelectric separator, particles are charged through contact electrification which can be carried out through rubbing among the particles or through contact between the particles in an operating medium. Corresponding to their specific electrical properties, particles having different work functions assume different charges when brought into contact. If two materials are brought into physical contact, it is believed that the material having the smaller work function transfers electrons to the material having the larger work function, with the result that the material having the smaller work function will acquire a positive charge and the material having the larger work function will acquire a negative charge. Thus, a pair of different particulate materials can be tribocharged with opposite signs, and the particles can then be separated by passing them between differentially charged electrodes.
The dielectric strength of a material increases as its work function increases, such that the greater the dielectric strength of a material, the greater its work function is. Triboelectric separation is generally suitable for separating different dielectric materials from each other.
As the triboelectric charging process generally creates relatively weak charges, the materials to be separated often must be pretreated in the form of heating or surface active reagent treatment in order to allow the particles to gain a sufficient charge. Pretreatment is not always practical or cost effective. An example of a triboelectric separator is disclosed in U.S. Pat. No. 5,118,407 issued to M. H. Beck et al.
Although there are a number of triboelectric separators and a number of corona separator designs, the existing electrostatic separators have not made use of both triboelectric charging and corona charging to effect separation of dielectric materials from dielectric materials or of dielectric materials from conductive materials. It is thus desirable to provide a method of electrostatic separation and a corresponding separator apparatus for separating different dielectric materials from each other, or dielectric materials from conductive materials, which employs both triboelectric and corona charging. It is also desirable to provide an electrode structure which can simultaneously provide a corona charging effect and a uniform electrostatic separation field.